It is known in the art to design a writing instrument, such as a pen, which has an external shell in which a refill is housed. It is also known in the art to join the refill to the shell using bearings such that the refill is rotatable relative to the shell. It has been speculated that by using a rotatable refill the ink may be more uniformly applied to the paper.
It is further known in the art to have a pen wherein the shell has a longitudinal axis and the rotatable refill is movable along the longitudinal axis between first and second positions. In the first axial position, surfaces of the rotatable refill and the external shell cooperate to limit the relative rotational motion between the refill and the shell. In the second position, the rotatable refill is capable, under certain conditions, of rotating freely relative to the shell such that a preferred rotational orientation state is maintained between the refill and the shell. Preferably, the insert is maintained in the second axial position until such time as the user wishes to write with the writing instrument, at which time the refill is moved to the first axial position.
Several structures have been proposed to permit the refill to assume the second axial position when the writing instrument is not being used, i.e. ink is not applied to paper. One suggestion has been to provide a shell having a first open end and a second closed end, a refill having a first end with a writing point and a second end and a spring. The spring is disposed between the second end of the refill and the closed end of the shell to bias the refill towards its second position. Alternatively, a tubular shell, a refill having a first end with a writing point and a second end, an annular plug, and a pin are provided. The plug is disposed within the tubular shell and the pin passed through the hole in the plug and secured to the refill. With the shell at an downward incline, gravity acts against the refill to urge the refill toward the second position, separation of the refill from the shell prevented by the cooperation of pin and plug.
Both of these structures have their drawbacks. In the first device, shoulders must be formed on the shell and the refill to prevent the separation of the refill from the shell as a consequence of the biasing force provided by the spring. The shoulders make the manufacture of the refill and the shell complicated and expensive, and also increase the complexity and expense of the assembly process. In the second device, while the pin eliminates the necessity of providing shoulders to retain the refill within the shell, the refill is only urged towards the second position if the shell is held relatively level or at a downward angle. If the shell is placed at an upward angle, the force of gravity actually encourages the refill toward the first position, thereby inhibiting the free rotation of the refill relative to the shell.